1. Field of the Invention
The present invention relates to a method for crystallizing tryptophan to obtain spherical crystals having a large particle size which are excellent in handling at separation and drying.
2. Background Art Relating to the Invention
In general, since crystals of tryptophan are in the form of fine powder and each of the crystals has a thin plate-like shape, there are problems that the crystals are difficult to flow or precipitate in slurry conditions, and furthermore, it is difficult to handle them at solid-liquid separation or drying after the separation. Therefore, as crystals of tryptophan, it is desirable to obtain spherical crystals having a large particle size. For that purpose, in methods for crystallizing tryptophan from a solution of water or a mixed solvent of water and an organic solvent, conditions, such as a temperature, a pH, a composition of solvents and the like, are controlled (e.g., JP-A-59-39857) or a third substance is added (e.g., JP-B-5-76463 and WO 90/09372) to control the particle size and the crystal form.
Among these methods, the method in which the operating conditions are improved achieves only slight enlargement of the particle size while maintaining the shape, and cannot improve the thin plate-like shape of tryptophan crystals. Also, the method in which a third substance is added can be expected to afford larger effects on both of the particle size and the shape than the method in which the operating conditions are improved. However, in the industrial operation, an additional apparatus and step for adding the third substance are newly required, so that there are problems that not only the method is economically disadvantageous but also complicated steps are required.